Method for wave-soldering printed circuit boards

ABSTRACT

A method is provided for wave-soldering printed circuit boards. In order to obtain wave-soldered printed circuit boards with smallest possible number of solder globules, after the passage on the wave of solder, the printed circuit boards are further artificially cooled at an approximately constant time-temperature gradient of 20 K/sec.

FIELD OF THE INVENTION

The present invention concerns a method for wave-soldering printedcircuit boards in which the printed circuit boards are artificiallycooled after passing through the solder wave.

BACKGROUND OF THE INVENTION

A method of this kind is disclosed in German Patent Specification DE 3611 180 C1, in which an apparatus is described for cooling solderingworkpiece carriers and soldered materials in soldering facilities bymeans of delivery of cool air. In the case of the known method, theprinted circuit boards are cooled on their underside, after passingthrough the solder wave, with air from nozzles that are arranged oneither side of the transport path. Smooth and also rapid cooling isthereby achieved, so that when cleaning subsequently occurs almost nocleaning agent evaporates; flux residues also essentially do notevaporate further.

It is moreover commonly known among those skilled in the art ofwave-soldering printed circuit boards that solder globules settle ontothe printed circuit boards during soldering; these are undesirable sincethey can lead to serious malfunctions in electrical or electronicdevices equipped with printed circuit boards of this kind.

Attempts have therefore already been made to prevent the formation ofsolder globules early on, during wave-soldering the printed circuitboards themselves, by using special solder resists; however,modification of solder resists in order to eliminate solder globules onprinted circuit boards is possible only to a limited extent, since thesolder resists must retain the ability to adhere to the copper coatingsof the printed circuit board. It is therefore not possible tomanufacture solder resists that completely eliminate adhesion of solder.

At present, therefore, it is accepted that solder globules will settleonto printed circuit boards during wave-soldering of the printed circuitboards. Since these solder globules must not, however, remain on theprinted circuit boards during assembly into an electrical or electronicdevice, the printed circuit boards are mechanically brushed off on thesoldered side in a production step subsequent to wave-soldering. To thisend, defined brush arrangements with specific brush materials arerequired, in order not only to reliably remove the adhesively attachedsolder globules from the solder resist on the printed circuit boards,but also to prevent damage to the printed circuit boards during thisoperation. The brushed printed circuit boards must then be subjected toa visual inspection to ensure that ultimately, only printed circuitboards that are free of solder globules are used.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forwave-soldering printed circuit boards with which printed circuit boardsfree of solder globules can be manufactured.

According to the invention, to achieve this object in a method of thekind indicated initially, cooling of the particular printed circuitboard to room temperature, achieving differential contraction of asolder resist mask usually located on the printed circuit board and ofthe solder applied onto the printed circuit board, occurs with anapproximately constant temperature gradient of approximately 20 K/sec.In the method according to the invention, the wave-soldered printedcircuit boards are therefore artificially cooled with the indicatedtemperature gradient after passing through the solder wave, whichimpedes the adhesion of solder globules onto the soldered side of theprinted circuit boards.

The invention is based on recognition of the following correlations:

During wave-soldering of, in particular, mixed-population printedcircuit boards, i.e. printed circuit boards that are populated with SMD(surface-mounted device) components and components equipped withconnecting wires, the flow characteristics of the solder wave aresubstantially disturbed by the components and their connections. Thiscauses the homogeneous solder wave to be separated by the flowimpediments as it passes along the printed circuit boards. The resultingseparated solder volumes are now particularly exposed to the oxidizingsolder atmosphere. Mixed tin-lead oxides are produced, which constitutesolids and cover the individual separated solder volumes. Because of thereduction in surface tension, they are particularly likely to becomeadhesively attached onto the solder resist mask of the printed circuitboards as solder globules. Solder resists consist, among other things,of inorganic fillers with polar properties, and have surface tensionvalues that are close to the surface tension values of tin-lead oxides.Relatively high adhesion forces therefore occur at the interfacesbetween the solder resist and the tin-lead oxides, and lead to adhesionof the solder globules.

The solder volumes that are separated by the flow impediments afterpassage of the solder wave are thermally at the level of the solderwave, and form oxidized tin-lead particles of spherical shape. Thethermal energy input during wave-soldering not only creates the jointbetween the solder and the parts of the printed circuit board to besoldered, but also leads to a substantial three-dimensional change inthe solder resist mask on the printed circuit boards. In a lowertemperature range the expansion of the solder resist mask isapproximately 60 to 80 ppm/K, and is considerably greater in the solderwave temperature range; the expansion of Sn60Pb40 solder, on the otherhand, is 23 ppm/K. The invention thus exploits the different thermalexpansion values of the solder resist mask and the tin-lead solder bydiminishing the adhesion surfaces between the solder resist and thesolder by means of an additional artificial cooling. The differentialcontraction of the solder resist mask and of the solder is exploitedhere in order to reversibly disrupt the adhesion surfaces formed betweenthe solder resist and solder during passage of the solder wave; in otherwords, a thermomechanical mismatch between the solder resist mask andsolder is deliberately produced by the additional artificial cooling, inorder to reduce adhesion forces.

In the method according to the invention, the additional artificialcooling of the printed circuit boards after passing through the solderwave can occur in various ways, for example with a cooling chamberthrough which the wave-soldered printed circuit boards pass. The methodaccording to the invention can, however, be performed particularlyeasily in terms of production engineering and with relatively littleoutlay if a gas flow, having a temperature which causes cooling with atemperature gradient of approximately 20 K/sec, is directed onto theregion of the zone where the solder wave breaks away from the particularprinted circuit board.

It is already known from DE Unexamined Application 28 52 132 A1 todirect a gas flow, which can contain air or an inert gas, against theunderside of the particular printed circuit board after the solder hasbeen applied onto the circuit board, but here the gas flow is intendedto impede the formation of solder short-circuits, drips, and bridges. Toachieve this satisfactorily in the case of the known method, factorssuch as the flow velocity, pressure, and temperature of the gas flowmust be adjusted to the particular conditions, which include the size ofand packing density on the printed circuit board. Since with the knownmethod the excess solder is blown away before the solder has solidified,a gas flow with preheated air is preferably used; consideration is alsogiven, however, to the use of a gas at ambient temperature.

It is moreover known, from the book "Weichloten in der Elektronik" Softsoldering in electronics! by R. J. Klein-Wassink, 1991, pp. 508 to 510,to direct a gas flow onto a wave-soldered printed circuit boardimmediately after wave-soldering. This is a hot-air flow whose purposeis to remove undesirable solder bridges between conductor paths in orderto achieve good solder joints. This hot-air flow, however, causesadditional solder globules, which must be removed by washing aftersoldering.

Particularly good operating results can be obtained with the methodaccording to the invention if an inert gas, which can preferably benitrogen, is used as the gas for the gas flow. The inert gas creates, inthe region where the printed circuit board emerges from the solder wave,a largely oxidation-free atmosphere which decreases the aforementionedoxidation of tin-lead particles due to the separation, thus helping todiminish solder adhesion on the solder resist.

It is also known, from the book by Klein-Wassink mentioned above, page508, to perform wave-soldering in an inert gas atmosphere with very lowoxygen concentrations, but here the entire wave-soldering process occursin an inert gas atmosphere, and additional artificial cooling is notundertaken. This leads to the creation of comparatively many solderglobules.

The method according to the invention can be performed in solderingfacilities of various designs. It is considered particularlyadvantageous, in terms of the manufacturing costs of a solderingfacility for performing the method according to the invention, if asoldering facility for wave-soldering printed circuit boards, having asoldering device and a cooling device for printed circuit boardsarranged after the soldering device in the transport direction of theprinted circuit boards, is used, in which according to the invention, anadditional cooling device for the printed circuit board, which effectscooling with the approximately constant temperature gradient of 20K/sec, is arranged after the solder wave generated by the solderingdevice in the transport direction of the printed circuit boards. Thisadditional cooling device can be configured in various ways.

It is considered particularly advantageous in terms of manufacturingcosts if the cooling device is a rod-shaped nozzle, extending transverseto the transport direction of the printed circuit boards, from which thegas flow is directed onto the particular printed circuit board afterpassing through the solder wave.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the part of a soldering facility of interest inconnection with the invention, for performing the method according tothe invention; and

FIG. 2 is a side cross-section view of a section of FIG. 1.

DETAILED DESCRIPTION

As shown in FIG. 1, a printed circuit board 1 to be soldered, having SMDcomponents 2 and components 3 with connecting wires, is located in atransport frame 4 that can be moved on a transport device 5 in thedirection of arrow 6. In the position depicted, the transport frame islocated just before a wetting device 7 in which, as printed circuitboard 1 moves over it, flux is applied onto the lower (in FIG. 1)soldered side of printed circuit board 1.

Before the printed circuit board is placed into transport frame 4, it isnot only populated with components 2 and 3, but also usually equippedwith copper conductor paths and covered with a solder resist on thelower (in FIG. 1) side at those points to which solder is not intendedto adhere to printed circuit board 1 or to components 2 and 3.

Once transport frame 4, with printed circuit board 1 to be soldered, haspassed over wetting device 7, it arrives in the region of a solderingdevice 8 which contains a container 9, depicted only schematically inFIG. 1, of liquid tin-lead solder 10. This solder--as is more clearlyevident from FIG. 2--is pumped upward by means of a pump (not depicted)in a delivery channel 11 in the direction of arrow 12, and thus forms asolder wave 13. Transport device 5 is aligned with respect to solderwave 13 in such a way that printed circuit board 1 to be soldered--asFIG. 2 once again clearly shows--passes through wave 13 so thatsoldering occurs.

Located after soldering device 8 in the transport direction (arrow 6) isnot only a first cooling device 14 with nozzles 15 and 16, but also anadditional cooling device 17 that is again depicted only schematicallyin FIGS. 1 and 2. Additional cooling device 17 contains substantially arod-shaped nozzle 18 that is equipped with individual outlet openings 19for a flow of inert gas. The orientation of nozzle 18 and its openingsis such that the inert gas, preferably nitrogen, is blown into region 20behind solder wave 13 against printed circuit board 1. The nitrogen isat a temperature no more than room temperature, so the gas flow resultsin an additional artificial cooling of printed circuit board 1 afterpassing through solder wave 13. When cooling occurs with atime/temperature gradient of approximately 20 K/sec, printed circuitboards are produced that have essentially no further solder globules onsoldered side 21.

Rod-shaped nozzle 18 can be arranged rotatably about its longitudinalaxis, so the additional cooling system can be adapted to differentprinted circuit board heat capacities.

We claim:
 1. A method for wave-soldering a printed circuit board, saidprinted circuit board including a solder resist mask thereon, comprisingthe steps of:passing the printed circuit board through a solder wave toapply solder thereon; artificially cooling the printed circuit boardafter passing it through the solder wave to cool the printed circuitboard to room temperature to achieve differential contraction of thesolder resist mask and of the solder applied onto the printed circuitboard with an approximately constant temperature gradient ofapproximately 20 K/sec.
 2. The method of claim 1, wherein a gas flow,having a temperature such that cooling occurs with the approximatelyconstant temperature gradient of 20 K/sec, is directed onto the regionof the printed circuit board emerging from the solder wave.
 3. Themethod of claim 2, wherein the gas is an inert gas.
 4. The method ofclaim 3, wherein the inert gas is nitrogen.
 5. A soldering facility forwave-soldering printed circuit boards, comprising:a soldering device forgenerating a solder wave to apply solder to the circuit boards; acooling device for cooling the printed circuit boards arranged after thesoldering device in the transport direction of the printed circuitboards; and an additional cooling device for the printed circuit boardsfor cooling the printed circuit boards at an approximately constanttemperature gradient of 20 K/sec arranged after the solder wavegenerated by the soldering device in the transport direction of theprinted circuit boards.
 6. The soldering facility of claim 5, whereinthe additional cooling device comprises a rod-shaped nozzle extendingtransverse to the transport direction of the printed circuit boards.